Method and apparatus for code division switching

ABSTRACT

Time division multiplexed input signals (i.e., separated into time slots) are spread in each time slot with a destination code. This spreading is applied for destination identification as opposed to signal information modulation. Each spreading signal is associated with a particular destination for a particular time slot. These spread signals are then combined on a code division bus. Output signals for retransmission to the destinations are recovered from the bus by application of despreading codes. Each despread signal has a destination code which appears in its individual time slot. In a variant code division multiplexed signals are directed to a destination by use of time slot interchanger to achieve time division switching to route the CDMA multiplexed channels.

FIELD OF THE INVENTION

This invention is concerned with signal switching technology and inparticular the use of code division switching (CDS) techniques in timemultiplexed switching systems for accomplishing switching and with thecontrol of such switching. It is specifically concerned with controlmechanisms for such a switch used in time division multiplexedcommunications systems.

BACKGROUND OF THE INVENTION

Code Division Switching (CDS) has been priorly discussed with relationto performing switching functions concerning processing CDMA modulatedsignals on board a satellite in communication systems using CDMA (CodeDivision Multiple Access) transmission technology. Examples of CDS havebeen disclosed in the U.S. Pat. No. 5,815,527 and in the U.S. Pat. No.5,995,497. These references disclose a switching technology for CDMAmodulated user signals in which multiple CDMA encoded beam signalsreceived by the switch are further CDMA encoded with the individualcomponent user signals of the beam which is also CDMA encoded. Encodingsignals (e.g., PN; Walsh; Quadratic; etc. codes) are applied to thebeams and user signals (i.e., uplink in the instance of a satelliteswitch). These signal codes are carefully selected to preventintersignal and interbeam interference from exceeding an acceptablelevel. At the switching level the user signals and beams are despreadrecovering the base signals and/or channels. These signals arerecombined into CDMA modulated beams in which the individual usersignals of any beam have a destination in common with that of the beamas defined by the applied spreading codes.

SUMMARY OF THE INVENTION

Therefore in accord with principles of the invention time divisionmultiplexed input signals (i.e., separated into time slots), in oneexemplary embodiment, are spread in each time slot with a destinationcode. This spreading is applied for destination identification asopposed to signal information modulation. Each spreading signal isassociated with a particular destination for a particular time slot.These spread signals are then combined on a code division bus. Outputsignals for retransmission to the destinations are recovered from thebus by application of despreading codes. Each despread signal has adestination code which appears in its individual time slot. In anotherexemplary embodiment a time multiplex switch directly switches CDMAsignals. In a variant of this a time slot interchange is used.

Code division multiplexing and/or code division used for switching timemultiplexed signals is a technology utilizing the code spreadingproperties of code division as a technique of switching traffic channelsand/or beams and individual signals (i.e., selecting point-to-point andpoint-to-multipoint paths for a particular channel/tine slotted signalto follow).

In an exemplary method of using code division for switching purposes,where a plurality of individual user signals are included in a compositebeam, Each user signal within each beam is assigned a set of orthogonalcodes which identifies that user. Pseudorandom (PN) codes are assignedto the beams for purposes of providing a cover code and suppressinginterference between the beams. Another set of orthogonal codes isadditionally provided for each beam for the purpose of providingorthogonal isolation between beams (i.e., nee3ded if a large pluralityof beams are transmitted). In the process of spreading the PN codes andorthogonal user codes are applied at one rate. A process ofoverspreading uses a spreading code having a higher rate than the basicspreading rate. The various codes enable identification of user signalsand beams and further allow extraction of individual user signals. Hencebeams can be constructed of user signals having a destination in commonwith the beam. The coding is for providing signal identity and identityof signal destinations.

A Code Division Switch (CDS), embodying the principles of the invention,includes a control unit that controls switch assignments based onsignaling information received during establishment of a call. Upon thearrival of a call a user sends a request to a control unit of the CDS tomake a switch assignment. The switch records all requests receivedwithin a defined time frame and applies an algorithm, which assigns theincoming and outgoing traffic channels. These assignment decisions aretransmitted back to the users whereby each user transmits data on theassigned traffic channel. The CDS provides the circuits for trafficrecovery and respreading to route each call in accord with theassignment decisions.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A, 1B and 1C are block diagrams schematically displayingapplications of a Code Division Switch (CDS) architecture;

FIG. 2 is a block schematic of an exemplary Code Division Switch used ina time division multiplexing application;

FIG. 3 is a block schematic of another exemplary Code Division switchused in a time division multiplexing application;

FIG. 4 is a block schematic of yet another exemplary Code Divisionswitch used in a time division multiplexing application;

FIG. 5 is a schematic of still another exemplary Code Division switchused in a time division multiplexing application;

FIG. 6 is a schematic of a sub circuit used in the various Code divisionSwitches.

DETAILED DESCRIPTION

Code division, as a switching technique, has been used with CDMAmodulated signals in many applications. It is readily applicable to CDMAmodulated signals since the varied signals use a common frequency bandwith the CDMA coding capturing information for each individual channel.It was extended to switching CDMA modulated signals aboard a satelliteby the use of over spreading to supply a destination code, which wasused to combine signals having a common destination into a beam ofsignals directed to the particular destination. This switching techniqueis combined herein with the switching of signals having time divisionmodulation.

Code division switching may be utilized in many applications includingsatellite switching as well as terrestrial applications. Suchterrestrial applications include base station switching in mobile andfixed wireless systems; switching in wired and cable systems. Thesevarious applications are shown in the FIGS. 1A, 1B and 1C. The satelliteswitch 101 in FIG. 1A includes an exchange node (i.e., code divisionswitch) and operates to switch uplink signals into downlink beamsdirected to particular ground stations. A wireless system shown in theFIG. 1B uses a code division switch (i.e., exchange node) in a basestation 103 to interconnect various radio distribution points 105. Acable communication system shown in FIG. 1C uses a code division switch107 (i.e., exchange node) to interconnect various head ends 109 and apublic Switched telephone network (PSTN) 111. In each of these examplesthe Exchange node is using time multiplexed switching in combinationwith code division techniques.

A time multiplex switch directly switching input CDMA signals achievestime multiplexing of CDMA signals. In this particular switchingarrangement, shown in FIG. 2, the input and output ports of the switchare time multiplexed and the individual signals are code divisionmodulated. The switch 201 comprises a switch matrix 203, which is a timemultiplex switch (i.e., TMS). The code division modulated uplink inputs,to be switched are supplied to input links, which in the example includereceivers 205-1 to 205-N, functioning for receiving and despreading CDMAsignals. After despreading all the despread signals are applied to inputports of the TMS 203 where they are time multiplexed and switched androuted to an appropriate output port of the TMS 203. These outputsignals are time demultiplexed and by action of the transmitters 211-1through 211-N are spread and code modulated for downlink delivery totheir destinations. Design of this switch needs to consider thenecessary time-to-time multiplex the input and output ports of the TMS203.

An improved switching system as shown in the FIG. 3 shows in detail ause of a time slot interchanger to achieve time division switching toroute CDMA multiplexed channels. The incoming CDMA signals are despreadand the data symbols are time multiplexed. Time slots, in coupling aninput to an output, are determined by a time slot interchanger (TSI)303. This TSI 303 enables a coupling of input receiving spreaders 305 tooutput transmitting spreaders 307 with less time delay than atraditional time division multiples switch. Time slots are rearranged ineach frame of the TSI to interconnect incoming time slots with timeslots so that an input signal is properly directed to a desireddestination. Time slot interchangers operate to exchange the positionsof time slots in a frame between an input and output port. These devicesare a well known and widely used in data processing and transmission anda detailed description thereof is not believed necessary.

Input beams of a plurality of CDMA signals are received at CDMAreceivers 311-1 through 311-N. Each receiver converts the receiver RF toBB or base band frequencies and despreads the incoming beam of CDMAsignals in beam despreaders 313-1 through 313-N. These despread beamsare applied to buses 315-1 through 315-N which is coupled to a pluralityof user signal despreaders 319-1 through 319-N. The output of eachdespreader is demodulated and applied to the TSI 303. TSI 303 rearrangesthe time slots in each frame so that each outgoing signal is directed toa time slot corresponding to the signal destination. The despreadsignals are demodulated processed at a sampling rate R_(s) and assignedto input time slots and assigned to a particular beam and coupled toappropriate time slots for sourcing an outgoing beam. Timing of theoutput slots and the appropriate respreading is controlled by a controlcircuit at input 339. The control unit assures that output timeslots areproperly aligned with the appropriate respreading to direct theresulting beam to tits destination.

Output signals of the TSI 303 are applied to output transmitters 321-1through 321-N. Each transmitter respreader includes modulation andspreading circuits 327-1 through 327-N to respread the signals. Theserespread signals are applied to a summing circuits 329-1 through 329-Nto form beams. These beams are each spread for transmission purposes bythe beam spreading circuit 331-1 through 331-N. The spread beam ismodulated to RF for transmission by the modulation circuits 333-1through 333-N.

Time slot interchangers have a size limit on the number of cross pointconnections that may be used. This may be addressed in large systems bycombining space switching with multiple time slot interchangers in amulti stage system. One suitable arrangement contemplates a timedivision stage followed by a space division stage (e.g., providing fixedphysical connections) which is then followed by a time division stage.

In an alternative arrangement joining time multiplexing with codedivision switching, time multiplexed or time divided input signals areswitched by a code division switch into time slots of a time multiplexedor divided output signal. A code division switch is used instead of atime slot interchanger. A block schematic of a Code division Switch usedin this application, as shown in FIG. 4, includes input ports 401-1 to401-N which accepts frames including multiple time slots and includedtime division multiplexed user signals in each time slot. The frames areall spread by application of an orthogonal spreading code W_(n) inspreaders 403-1 through 403-N and applied to a summing circuit 405 whereall the spread signals are combined. The spreading codes applied aresynchronized with the time slots and define the time slot destination ofeach of the time multiplexed frames. The combined signals are applied toa code division bus 407. Each of these code modulated signals, in thecode division bus, has been spread so as to be orthogonal to one anotherand hence be non-interfering.

Coded modulated signals to be transmitted are recovered from the codedivision bus and despread in despreaders 409-1 through 409-N and arefurther processed by the filter integrators 411-1 through 411-N torecover the time modulated frame for transmission and the relateddestination.

The spreading and despreading signals are supplied to the spreaders anddespreader on a time slot to time slot basis. The spreading codes W_(n)and W_(N) are supplied by a separate control unit discussed hereinbelow.

Many code division switch embodiments have been discussed in theaforementioned patents and may be used herein and are incorporatedherein by reference. An alternative code division switch embodiment mayprocess individual I and Q signals. One such alternative arrangement isshown in the FIG. 5 in which each set of the I and Q components ofincoming beams are spatially separated and recombined into differentcombined sets for code division processing. In this arrangement a codedivision switch receives incoming beams which are applied to a RF toBase Band converter 501-1 through 501-N. The I and Q output of eachconverter is applied to respreading and overspreading circuitry 503-1through 503-N. The I and Q signals are separated and applied todifferent summing circuits 507-1 through 507-N in reverse application.The summed I and Q signals are applied to the code division busses 511-1through 511-N. The bus output is tapped into a plurality of trafficchannel recovery circuit and respreading circuits (TCR&RC) 513-1 through513-L equal to the number of channels (L) included within each beam. TheTCR&RC circuits are coupled to summing circuits 515 and the I and Qoutputs are applied to beam recovery and overspreading circuits 517which recover the downlink beam. By use of the spreading code, the downlink beams are each identified for direction to its destination.

A critical component in these switching arrangements is the trafficchannel recovery and respreading circuitry (TCRC). Suitable circuitryfor this purpose de-over-spreads despreads, respreads andre-over-spreads. Circuitry in FIG. 6, for this purpose has ade-over-spreading section 601, a de-spreading section 603, aRe-spreading section 605 and a re-over-spreading section 607. Thevarious spreading and despreading operations are useful in supplying thecoding to control user signal and beam destinations and to identify usersignals chosen for each destination. The exemplary de-overspreadingcircuit 601 illiterates parallel paths although the illustration is onlyexemplary (i.e., single path circuitry may be used). It includes aninput to mixers 613-1 and 613-2, responsive to an orthogonal code W_(n),which in combination with a subsequent integrating filter 615-1 and615-2, de-over-spreads the incoming signal to identify particular beams.This is followed by section 603 that has similar components such asmixers 621-1 and 621-2 and integrating filters 623-1 and 623-2. Itdespreads for the purpose of recovering original user informationsignals and identifying the uplink users. Codes, such as W_(k) (i.e.,users uplink orthogonal code), are used to despread the de-over-spreadsignal. Respreading, to identify a downlink user occurs in section 605that includes a plurality of serially connected mixers 625-1 and 625-2;627-1 and 627-2; and 629-1 and 629-2. The applied codes are W_(m) (userdownlink code), and C_(i) and W_(j) (beam downlink codes). The spreadsignals are applied to the over-spreading section 607, which applies thecode Wn to the sections mixers 631-1 and 632-2. This overspreading withan orthogonal code Wn achieves orthogonal separation between the systemtraffic channels allowing combining and summing operations on thesignals. Further discussion of spreading and despreading circuitry isnot believed necessary since such circuitry is well known in traditionalapplications of CDMA coding which is discussed in the US patentsdiscussed herein above.

Various techniques of arranging and rearranging the components of theTCRC and other basic combinations to achieve specific CDM coding effectshave been disclosed. The details of such devices may be attained byreviews of the US patents cited above and whose teachings areincorporated herein by reference.

1. A method of switching a signal comprising: coupling an input tooutput by plural stages of switching; processing the signal switching bycoding techniques in one stage of the plural stages of switching;processing the signal switching by timing techniques in another stage ofthe plural stage of switching; coordinating the coding techniques andtiming techniques to couple a signal at an input port to a predeterminedoutput port: despreading a code division signal; time multiplexing thedespread code division signal; and respreading the time multiplexedsignal.
 2. A method of switching a signal comprising: coupling an inputto output by plural stages of switching; processing the signal switchingby coding techniques in one stage of the plural stages of switching;processing the signal switching by timing techniques in another stage ofthe plural stages of switching: coordinating the coding techniques andtiming techniques to couple a signal at an input port to a predeterminedoutput port; spread coding a time multiplexed signal; code divisionswitching the spread coded signal; and time multiplexing the codedivision switched signal.
 3. A method of switching a signal comprising:coupling an input to output by plural stages of switching; processingthe signal switching by coding techniques in one stage of the pluralstages of switching, wherein the processing in one stage comprises:switching by timing techniques comprises a time slot interchange step;and switching by coding techniques comprises a code division step;processing the signal switching by timing techniques in another stage ofthe plural stages of switching; coordinating the coding techniques andtiming techniques to couple a signal at an input port to a predeterminedoutput port; combining signals into beams and encoding the beams withoverspreading techniques to identify a destination; and codingindividual signals to identify a particular user.